Really not convinced that it is? I have 63A worth of heating, a 13A immersion heater, a range cooker rated at >60A. I appreciate diversity should obviously be taken into account but it seems too close to the bone to me? Our new property is very similar - the heating in that is (going to be) Air Source, so more efficient, but still >50A at full load as it is a much larger area, plus water heating, dryer, washing machines, ovens/hobs etc.
I have never had an actual issue, granted, but it does worry me a little in cold weather when the heating is fully firing and then my wife also turns on dryers and ovens etc. EVs on paper don't work on a 100A supply with electric heat as far as i can tell you'd have to be mega careful with timing?
(Not a qualified electrician, but i did the whole installation here and am doing in our new property, uni was electrical & electronic engineering).